Plastic hollow structure plates, such as flute-shaped plastic cardboards (product name: Danplate manufactured by Ube-Nitto Kasei Co., Ltd.), corrugated plastic cardboards, plastic structure plates in which columnar independent air compartments are formed (product name: Plapearl manufactured by Kawakami Sangyo Co., Ltd.), are lightweight and have excellent water resistance, heat resistance, chemical resistance and other properties, and thus have conventionally been used in various applications such as building panels, containers, various boxes, and interior materials for, for example, houses, buildings, offices, and vehicles (e.g., see JP 2000-326430A as a honeycomb structure plate).
Among these, hollow structure plates in which columnar independent air compartments (hereinafter, referred to as “hollow protrusions”) are formed are known to have no difference in strength between the vertical and the horizontal directions, compared with corrugated plastic cardboards or flute-shaped plastic cardboards.
Such a structure plate can be obtained by molding a thermoplastic resin sheet under a reduced pressure. In this structure, when it is attempted to increase the height of a hollow protrusion to increase the thickness, the wall portion constituting the hollow protrusion turns into a film, so that the strength cannot be maintained. If the thickness of the resin sheet is increased in order to solve this problem, then the weight is inevitably increased and the property of being lightweight is impaired. Therefore, a technique disclosed in JP 2000-326430A has been developed as a conventional art of the present invention.
In this technique, a plurality of hollow protrusions are projected in each of a pair of resin sheets, and the resin sheets are fused with the hollow protrusions facing each other, so that a hollow structure plate is obtained. By attaching the two sheets, the thickness can be made twice as large as the conventional thickness while the strength is maintained. Subsequently after the formation of the hollow protrusions and the attachment process, a smoothly planed board or the like is laminated on the opposite surfaces of the two resin sheets, and thus a plate material product having a lightweight hollow structure can be obtained.
A method for manufacturing this structure, which is disclosed in the above-mentioned publication, is as follows. Two resin sheets extruded from a T-die are supplied between a pair of emboss rollers in which a multitude of pins are projected from their outer circumferential portions, and the pressure inside the rollers is reduced and evacuated, so that the two resin sheets are molded so as to be pin-shaped under a reduced pressure. At the same time, the pins are brought in contact with each other with the rotation of the rollers so that the end faces of the hollow protrusions are thermally fused to become integral. In this state, the sheets are taken up by a take-up roller, and thus an integral hollow structure plate can be obtained.
However, the above-described manufacturing method has the following technical problems in terms of the shape or the properties, and cannot be adapted for manufacturing in practice.
First, often, the sheet cannot be attached onto the roller simply by evacuating the roller, and evacuation loss occurs, so that when forming hollow protrusions having a large height, molding is impossible.
Second, when the rollers are heated to a temperature at which the hollow protrusions of the resin sheets can be molded, the resin sheets get attached to the rollers, which makes mold release difficult. Even if mold-release is performed successfully, the shape is transformed because the temperature of the hollow protrusions is at least a melting point or more. On the other hand, if the molding temperature is too low, then the bottom surfaces of the hollow protrusions cannot be fused and joined together, even if they are in contact with each other. In such a case, the resin sheets are detached from each other at the joined portion when a bending load is applied to the obtained hollow structure plate, and thus the rigidity is decreased. Therefore, strict control of the temperature becomes necessary in order to obtain high quality hollow structure plates.
Furthermore, when the difference in the size (diameter ratio) between the upper base and the lower base of each pin is small, not only does the mold-release property deteriorate, but also the following problem occurs: in the molded hollow structure plate, the amount of resin for the hollow protrusions is increased and the balance with the liner portion in which no hollow protrusion is formed collapses, so that webbing occurs and thus the formativeness deteriorates.
Furthermore, the bending elasticity gradient of the hollow structure plate is improved as the interval between the pins is smaller, but simply reducing the interval between the pins causes webbing in the hollow structure plate more easily, especially when the difference in the size between the upper base and the lower base of the pin is small.
Furthermore, such a hollow structure plate has a poor sound absorbency, and it is necessary to attach a porous sheet-shaped member such as an urethane foam sheet, a non-woven fabric, and a woven fabric in order to improve the sound absorbency. Furthermore, in a commonly used sound absorbing material, the sound absorbency depends greatly on the thickness thereof, so that for example, as the thickness of the porous sheet-shaped member becomes smaller, the sound absorbency, especially in the low or medium frequency region, becomes poorer. Moreover, rock wool, plaster boards and the like that are used as interior materials for houses such as ceiling materials or wall materials are lightweight and have excellent sound absorbency and thermal insulation performance, but the rigidity and water resistance are poor.
The present invention solves the above-described technical problems, and an object thereof is to provide a method and an apparatus for manufacturing a hollow structure plate that allow hollow protrusion processing and melt joining of two thermoplastic resin sheets that have been extrusion-molded to be performed reliably in a short time, and that facilitate the temperature control.
Furthermore, the present invention solves the above-described technical problems, and an object thereof is to provide a technique that allows hollow protrusion processing and melt joining of two thermoplastic resin sheets that have been extrusion-molded to be performed reliably in a short time and facilitates the temperature control, and that also enables manufacturing of a hollow structure plate having good flexural properties.
Furthermore, the present invention has an object of providing a lightweight hollow structure plate having excellent strength, rigidity, heat resistance, and water resistance and high sound absorbency with an appropriate thickness without attaching a porous sheet-shaped member such as an urethane foam, a non-woven fabric, and a woven fabric. Furthermore, the present invention has an object of providing a hollow structure plate having high sound absorbency throughout the audible range by combining the above-described hollow structure plate and another sound absorbing material, without canceling out the effect of one another.